BOISE, Idaho. — The latest Republican to surge in polls, Rick Santorum is trying to turn his newfound strength into something lasting.

Curious Republicans now pack his rallies. Supporters have funneled nearly $4 million to his formerly empty campaign account over the past seven days. And his staff is plotting an aggressive strategy to challenge Mitt Romney in Romney's native Michigan and beyond.

But things don't look so strong just beneath the surface.

Santorum is underfunded and outmanned. He's still lacking in organization, a month and a half into the primary season. And, after he won three contests in a single day last week, his opponents — on the right and the left — have begun their own efforts to tear him down.

An upbeat Santorum declared "We're building" in a brief interview in Tacoma, Wash., on Tuesday before heading to Idaho for campaign events. "We've got a great volunteer base. In some states we're going to have staff. Other states we aren't. We're going to use volunteers."

His challenges were on display this week in Tacoma, where hundreds of supporters waited on cold, wet cement stairs in the dark to see the Republican presidential candidate with whom they're barely familiar.

"I don't know a lot about him, except I know he's more conservative than some of the other candidates like Mitt Romney," said Tanya Franklin, a 54-year-old airline reservationist, who says she'll probably vote for Santorum in her state's March 3 caucuses.

The former Pennsylvania senator has surged to a virtual tie with Romney in nationwide polling following his surprising sweep in Colorado, Minnesota and Missouri last week. But, as Franklin suggests, his popularity may have less to do with who he is than who he isn't. Santorum is not Romney. And with Newt Gingrich's recent decline, that's enough for some conservatives — at least for now.

Santorum had 30 percent support to 28 percent for Romney in a national poll released this week by the Pew Research Center. But the same poll said 31 percent of all adults had never heard of or couldn't rate him. That's a significantly higher number than for Romney, Gingrich, or Ron Paul. Even among Republicans, one in five told Pew they didn't know enough about Santorum to rate him.

The long-time front-runner for the nomination, Romney has deployed surrogates such as a former Santorum Senate colleague, Jim Talent of Missouri, to attack Santorum's support for earmarks in Congress. The conservative Club for Growth has been equally critical. And Romney has been aggressive on the campaign trail, suggesting in recent days that Santorum and Gingrich represent the kind of overspending Washington insiders the tea party abhors.

At the same time, left-leaning groups such as the Center For American Progress and Emily's List are going after Santorum's comments on women. A staunch social conservative, Santorum has been critical of women serving in combat and sometimes in the workplace.

"Sadly the propaganda campaign launched in the 1960s has taken root," reads a passage in Santorum's book. "The radical feminists succeeded in undermining the traditional family and convincing women that professional accomplishments are the key to happiness."

"These things that Rick Santorum is attacking are broadly supported by women and American families," said Tara McGuinness of the Center for American Progress. "It isn't 1952. Most American families have two working parents."

Santorum says he's not going to sit back and just take such shots.

On Tuesday, he began running ads on Fox News Channel in Michigan. It was a signal to supporters — and to donors — that Santorum planned to contest the state where Romney grew up and his father served as governor.

Look for Santorum to emphasize his message on manufacturing revival, especially in hard-hit Michigan. He plans a Thursday economic speech in Detroit, and his advisers see an opening to use Romney's words against him — especially Romney's 2008 New York Times op-ed titled "Let Detroit Go Bankrupt."

Sensing vulnerability, Romney tried to pre-empt that in an op-ed Tuesday in The Detroit News. He argued that the government should sell its share in automakers and return the profits to taxpayers.

At the same time, Santorum's advisers are bracing for an onslaught from Romney. They were largely spared in the last trio of states — Minnesota, Missouri and Colorado.

"We fully expect his search-and-destroy methods to be put on display. That's his M.O.: instead of focusing on his own record, their first inclination is to tear down his opponent," said Hogan Gidley, Santorum's communications chief.

But there are limits to what Santorum's little team can do.

He refuses to hire a pollster and pledges to campaign from his gut. He has brought on veterans from Mike Huckabee's orbit, including the top spokeswoman for Huckabee's 2008 presidential bid and Michele Bachmann's 2012 race. Other aides, too, are coming aboard as Santorum's strategy meetings have grown from just one state to a handful of competitive races.

Yet he lacks a headquarters to have those meetings. Often, Santorum's top aides confer over conference calls or Skype.

The disorganization caused problems at the Tacoma event, which was held at an outdoor venue adjacent to a camp site of the local Occupy protesters. Lacking the staff to handle such logistics on his own, Santorum had left the planning to the state GOP.

A confident Santorum took the stage as supporters chanted, "We pick Rick!"

"You have very good taste. Thank you," Santorum responded.

But in a matter of minutes, his supporters were overshadowed by shouting from liberal protesters who packed the event. Two were dragged away by police in the midst of his speech.

A week after his trio of victories, Santorum said he's raised nearly $4 million since then and expects to hit that mark soon.

Romney, by contrast, averaged more than $2 million a week over the last three months of 2011. Santorum raised less than $1 million over the entire quarter.

It's unclear how long Romney will wait to take an anti-Santorum message to the airwaves. His allies with the Restore Our Future super PAC already have bought time in several states and plan to go after Santorum aggressively.

Romney aides point to three vulnerabilities beyond Santorum's support for some earmarks.

They note that Santorum was one of a handful of Republicans who voted to confirm Sonia Sotomayor to the federal circuit court in 1998. Sotomayor was nominated by President Barack Obama, and confirmed by the Senate, to the U.S. Supreme Court in 2009.

Santorum also faced criticism during the New Hampshire and South Carolina primary campaigns for opposing right-to-work legislation, an issue Romney aides expect to re-emerge.

They also expect Santorum's endorsement of then-Pennsylvania Sen. Arlen Specter over Rep. Pat Toomey in the 2004 GOP Senate primary to become an issue. Santorum said in a video appeal for Specter, a social moderate, that the senior Republican senator was "with us on the votes that matter." In 2009, Specter changed his party affiliation to Democrat.

Gingrich won't make things easier for Santorum either.

Despite falling in the polls, the former House speaker says he's the strongest Romney alternative.

The National Review, an influential conservative magazine, published an editorial calling on Gingrich to step aside and endorse Santorum. But Gingrich this week called the article "silly" and said he had no intention of abandoning the race.